The present invention relates to liquid crystal display (LCD) devices and, more particularly, to a novel liquid crystal display device comprising a pair of birefringent films to provide a black-off state.
An LCD device typically comprises a multiplicity of picture elements, or "pixels", formed between a pair of flat panels (usually a glass substrate and a cover glass) sealably containing a quantity of twisted nematic liquid crystal material. If the LCD device is to operate by reflected light, only one of the panels need be transparent and the other panel will be formed with a reflective surface. If the LCD device is to be light transmissive, then both flat panels should be transparent. A detailed description of an LCD device structure and operation is disclosed and claimed in U.S. Pat. No. 4,646,424, issued Mar. 3, 1987, assigned to the assignee of the present invention, and incorporated herein in its entirety by reference.
Active matrix LCD pixels are usually arranged in uniform columns and rows to form an X-Y matrix structure. A semiconductor switch, such as a thin film field-effect transistor (FET) or the like, is integrally formed with each pixel to control the operation of that pixel in the display. Electrical communication with the individual pixel FETs is accomplished by a plurality of X-address lines or scan lines and a plurality of Y-address lines or data lines which are both formed during device fabrication. The scan lines are usually connected to the gate electrodes of the pixel FETs, the data lines are usually connected to one of the source or drain electrode, and the other of the source or drain electrode is connected to a pixel electrode of each pixel. Thus, a voltage of proper polarity and magnitude applied to a scan line will cause the FETs in the row corresponding to the scan line to "switch-on" to a conducting state. If a data voltage is applied to a data line while an FET in the column corresponding to the data line is in an "on" state, the alignment of the liquid crystal molecules is altered depending upon the magnitude of the data voltage applied between the pixel electrode and a ground plane electrode common to all pixels. The data voltage magnitude may be such as to: allow no light transmission through the pixel (off); allow maximum light transmission through the pixel (on); or provide an intermediate gray scale level of light transmission. Ideally, a pixel in the "off" state should completely block out any light transmission and look black for the display to exhibit the desired contrast and resolution.
The elongated molecules of the twisted nematic (TN) liquid crystal material will cause the polarization of light propagating through the LCD to rotate by an angle .theta. corresponding to the change in orientation or alignment of the liquid crystal (LC) molecules between the pair of flat panels. Each of the liquid crystal molecules has a longitudinal axis; the longitudinal axes of the LC molecules closest to the entrance panel (or glass substrate) will be oriented in one direction (for example .theta..sub.1 =0.degree.) and the longitudinal axes of subsequent LC molecules between the pair of flat panels will each be rotated an incremental angle (.DELTA..theta.) relative to the axes of the immediately proceeding molecules (.theta..sub.n =.theta..sub.n-1 +.DELTA..theta.). The last LC molecules closest to the exit panel (or cover glass) will then be oriented at an angle (for example .theta..sub.n =90.degree.) relative to the first LC molecules The angle between the first molecules and the last molecules is commonly known as the twist angle of the LC cell. For a thick LCD device having a twist angle of 90.degree., with linear entrance and exit polarizers, each having parallel polarization alignment, and each respectively placed adjacent the LCD entrance and exit panels, a dark output will be provided from each pixel that is in the "off" state. The entrance polarizer will pass or excite one of the two normal modes and block the other mode of the linearly polarized propagating light The LC molecules will rotate this mode 90.degree. as the light propagates through the LC cell and the exit polarizer, which is aligned parallel to the entrance polarizer, will block this rotated mode of the propagating light.
In thin LCDs, such as the fast displays required for color video, or in highly twisted cells (.theta.=270.degree.), such as the highly multiplexable super-twist displays, the "off" state is not black because the two normal modes of light propagating in the LCD are elliptically polarized as a result of the optical rotation and birefringence of the TN molecules; thus, both modes are excited and, when exiting the cell, cannot be completely blocked with a linear exit polarizer. The degree of ellipticity (.beta.) of the two normal modes of a LC cell is expressed by the equation: ##EQU1## where .lambda. is the light wavelength, .DELTA.n is the LC birefringence and p is the pitch of the twisted LC molecules. The pitch is directly proportional to the thickness (d) of the LC cell; for a 90.degree. twisted nematic cell, p=4d and the ellipticity equation becomes: ##EQU2##
The azimuth of the two normal modes of light propagating through the LC cell are each respectively parallel and perpendicular to the LC molecular direction; therefore, the two normal modes will be rotated by the twist angle of the LC cell.
In color LCDs, different colored light, each at a different wavelength (.lambda.), will, therefore, have a different elliptical polarization or ellipticity (.beta.) upon exiting the LCD. The color LCD can be designed with a special combination of LC birefringence (.DELTA.n), cell thickness (d) and cell twist angle (.theta.) to provide a minimum transmission of light at a particular wavelength, such as green light (.lambda.g=540 nm) but considerable light will leak through at the red (.lambda.r=640 nm) and blue (.lambda.b=470 nm) wavelengths to make the "off" state appear colored or not black.
Prior art devices have been proposed which utilize a single birefringent film (BF) disposed in an optical path after the LCD to produce a more neutral "off" state. These single BFs require precise orientation of the film to optimize the neutral color "off" state and also have retardations (i.e. the product of the birefringence, .DELTA.n', of the film and the thickness, d', of the film) greater than about 300 nm for optimum performance with LC cells of various pitch, birefringence and polarizer orientations. LCD devices with a single large-retardation-value BF are quite performance sensitive to polarization orientation, BF orientation and LC cell thickness. All of these parameters must be properly coordinated to optimize the dark output during the "off" state.
It is accordingly a primary object of the present invention to provide a novel liquid crystal display device which is not subject to the foregoing disadvantages.
It is a further object of the present invention to provide a liquid crystal display device with a black output "off" state for all wavelengths of colored light.
It is another object of the present invention to provide a liquid crystal display device which is insensitive to LC cell thickness, polarizer orientation, birefringent film orientation and viewing angle.
These and other objects of the invention, together with the features and advantages thereof, will become apparent from the following detailed description when read with the accompanying drawings in which like reference numerals refer to like elements.